Fix interaction of max_align_t and modules.

When building with modules enabled, we were defining max_align_t as a typedef
for a different anonymous struct type each time it was included, resulting in
an error if <stddef.h> is not covered by a module map and is included more than
once in the same modules-enabled compilation of C11 or C++11 code.

llvm-svn: 218931
diff --git a/clang/lib/Headers/module.modulemap b/clang/lib/Headers/module.modulemap
index 93e4d65..3c42477 100644
--- a/clang/lib/Headers/module.modulemap
+++ b/clang/lib/Headers/module.modulemap
@@ -170,3 +170,7 @@
     }
   }
 }
+
+module _Builtin_stddef_max_align_t [system] [extern_c] {
+  header "__stddef_max_align_t.h"
+}